1. Field of the Invention
The present invention relates to an X-ray mask for use in X-ray exposure and, more particularly, to an X-ray mask in which an X-ray transmitting thin film is improved.
2. Description of the Related Art
Recently, as the degree of integration of semiconductor devices has been increased, the extent of micropatterning of a circuit pattern of LSI elements which constitute a semiconductor device has been increased. In order to form a very fine pattern on the order of so-called subhalfmicrons, high-resolution exposure transfer techniques are essential. As one of these techniques, there is proposed an exposure transfer technique using X-rays having a wavelength by far shorter than that of currently widely used ultraviolet rays. To put such an X-ray exposure transfer technique into practical use, a large number of technical problems must be solved. Of these problems, the formation of an X-ray mask is of most concern.
FIG. 10 shows a representative sectional structure of an X-ray mask. The main portion of this X-ray mask is composed of a support frame 1, an X-ray transmitting thin film 2 serving as a mask substrate, and an X-ray absorber pattern 3.
Of these parts, the X-ray transmitting thin film 2 is required to have a sufficient transmittance of X-rays used in exposure, a sufficient radiation resistance against intense X-rays used in exposure, and a sufficient transmittance of visible light (wavelength 633 nm) used in an alignment between a mask and a wafer. The film 2 is also required to have a sufficient mechanical strength and a small tensile stress so that a fine X-ray absorber pattern does not cause a displacement.
In many cases, the X-ray transmitting thin film 2 is formed on a substrate, such as an Si wafer, which constitutes the support frame 1. For this reason, the step of forming the film on the substrate is an important step which determines the characteristics of the X-ray transmitting thin film 2. Note that unnecessary portions of the substrate are removed by etching.
Conventionally, BN, Si, SiN, and SiC, for example, have been examined as the material of the X-ray transmitting thin film, and a vacuum vapor deposition method, a sputtering method, a CVD method, and the like have been studied as the formation method of the film. However, it is difficult to obtain a film which completely satisfies the above conditions.
For example, the use of BN or SiN makes it difficult to form a film having a sufficient radiation resistance against intense X-rays used in exposure. Although Si is satisfactory in radiation resistance, a film having a high visible light transmittance is difficult to form by using this material. SiC is a substance which satisfies the above conditions comparatively well, but it has the following problem. That is, although SiC is a material originally having a high visible light transmittance, crystal defects are easily produced upon film formation using this material, and this makes it impossible to obtain a high visible light transmittance.
In order to solve the above problems, the use of a stacked composite film consisting of two or more layers of different types of materials has been examined. In this method, however, it is necessary to use different source gases upon film formation performed by a CVD method. In addition, a technique of using different film formation methods for a single material has been studied. In this technique, for example, an amorphous film is formed by a plasma CVD or ECR-CVD method on a polycrystalline film formed by a thermal CVD method, and this composite film is used. However, this technique requires a plurality of different types of film formation apparatuses.
As described above, it is difficult to obtain a sufficiently high visible light transmittance when SiC is used as the material of the X-ray transmitting thin film. In addition, the formation of a stacked composite film proposed as the X-ray transmitting thin film complicates the manufacturing steps and makes it difficult to easily improve the visible light transmittance.